With the recent technological developments, a large-sized lithium-ion secondary battery for power storage as well as for an electric vehicle has been utilized. Such large-sized lithium-ion secondary batteries for power storage are installed in houses, offices, premises of electric power companies, or the like, and an operation lifetime of 10 years is postulated.
For the large-sized lithium-ion secondary battery in which a usage lifetime of 10 years is expected, the battery state thereof needs to be monitored due to maintenance and management. For example, constant monitoring of temperature is important as information for determining the absence or presence of abnormality such as heat generation. In addition, monitoring of an available battery capacity value is necessary as information for determining the necessity or needlessness of a battery maintenance operation.
It is known that deterioration progresses with use in the lithium-ion secondary battery and the available battery capacity decreases. In a case where the battery capacity is reduced faster than assumed due to certain abnormality, or in a case where the battery capacity is reduced lower than the guaranteed capacity within a guaranteed period, maintenance or replacement is necessary.
As the simplest battery capacity monitoring method, a method of fully charging a battery and thereafter fully discharging the battery has been considered. Full charge and full discharge of the battery only for the monitoring of the battery capacity is a waste of electricity, and there is a problem in that a user cannot use the battery in the meantime. In a case of employing the method, it is preferable that the battery capacity is monitored through everyday use (charge and discharge) by the user. However, there is a problem in that for some users, it is rare that the battery is used to the point of full discharge. For example, there is a user who uses a battery while always ensuring a power of SOC=30% or higher. Here, SOC stands for State of Charge and means a charge rate. In addition, there may be a case where, depending on the user, charge is newly performed while full discharge is not reached due to a small amount of power used in the daytime.
As another battery capacity monitoring method, a method of estimating a deterioration amount by analyzing the log data of a lithium-ion secondary battery, that is, a discharge curve or a charge curve may be considered. However, the charge or discharge curve needs to be acquired with high accuracy to a certain degree, and even in this method, full discharge after full charge or charge after full discharge is preferable. Therefore, the same problems as those in the above-described method are present.
As another battery capacity monitoring method, a method of estimating an amount of reduced battery capacity by measuring an impedance or resistance value may be considered. However, in this method, a measurement mechanism other than mechanisms for measuring current, voltage, and power needs to be provided, and this causes an increase in costs.
In Patent Document 1, a method is disclosed of estimating three parameters including the active material retention rate at each of positive and negative electrodes and a variation capacity corresponding to the compositions of the positive and negative electrodes, and estimating the full charge capacity of a deteriorated battery using the estimated parameters.